Cold work steel

ABSTRACT

A cold work steel has the following chemical composition in weight-%: 1.25-1.75% (C+N), however at least 0.5% C 0.1-1.5% Si 0.1-1.5% Mn 4.0-5.5% Cr 2.5-4.5% (Mo+W/2), however max. 0.5% W 3.0-4.5% (V+Nb/2), however max. 0.5% Nb max 0.3% S balance iron and unavoidable impurities, and a microstructure which in the hardened and tempered condition of the steel contains 6-13 vol-% of vanadium-rich MX-carbides, -nitrides and/or carbonitrides which are evenly distributed in the matrix of the steel, where X is carbon and/or nitrogen, at least 90 vol-% of said carbides, nitrides and/or carbonitrides having an equivalent diameter, Deq, which is smaller than 3.0 μm; and totally max. 1 vol-% of other, possibly existing carbides, nitrides or carbonitrides.

This application is the U.S. National Phase of International ApplicationPCT/SE02/00930, filed 17 May 2002, which designated the U.S.

TECHNICAL FIELD

The invention concerns a cold work steel, i.e. a steel intended to beused for working a material in the cold condition of the material.Typical examples of the use of the steel are tools for shearing(cutting) and blanking (punching), threading, e.g., for thread rollingdies and thread taps; cold extrusion tooling, powder pressing, deepdrawing and for machine knives. The invention also concerns the use ofthe steel for the manufacturing of cold work tools, the manufacturing ofthe steel and tools made of the steel.

BACKGROUND OF THE INVENTION

Several demands are raised on cold work steel of high quality, includinga proper hardness for the application, a high wear resistance, and ahigh toughness. For optimal tool performance both high wear resistanceand good toughness are essential. VANADIS® 4 is a powder metallurgicalcold work steel manufactured and marketed by the applicant, offering anextremely good combination of wear resistance and toughness for highperformance tools. The steel has the following nominal composition inweight-%: 1.5 C, 1.0 Si, 0.4 Mn, 8.0 Cr, 1.5 Mo, 4.0 V, balance iron andunavoidable impurities. The steel is especially suitable forapplications where adhesive wear and/or chipping are the dominatingproblems, i.e. with soft/adherent working materials such as austeniticstainless steel, mild carbon steel, aluminium, copper, etc. and alsowith thicker work materials. Typical examples of cold work tools, wherethe steel may be used are those which have been mentioned in the abovepreamble. Generally speaking, VANADIS® 4, which is subject of theSwedish patent No. 457 356, is characterised by good wear resistance,high pressure strength, good hardenability, very good toughness, verygood dimension stability when subjected to heat treatment, and goodtempering resistance; all said features being important features of ahigh performance cold work steel.

The applicant also has designed a steel WO 01/25499, having thefollowing chemical composition in weight-%: 1.0-1.9 C, 0.5-2.0 Si,0.1-1.5 Mn, 4.0-5.5 Cr, 2.5-4.0 (Mo+W/2), however max. 1.0 W, 2.0-4.5(V+Nb/2), however max. 1.0 Nb, balance iron and impurities and having amicrostructure, which in the hardened and tempered condition of thesteel contains 5-12 vol-% MC-carbides, of which at least 50 vol-% have asize which is larger than 3 μm but smaller than 25 μm. Thismicrostructure is obtained by spray-forming an ingot. The compositionand microstructure affords the steel features which are suitable forrolls for cold rolling, including suitable toughness and wearresistance. Further, a high speed steel manufactured in a conventionalway by ingot casting is disclosed in EP 0 630 984 A1. According to adescribed example, the steel contained 0.69 C, 0.80 Si, 0.30 Mn, 5.07Cr, 4.03 Mo, 0.98 V, 0.041 N, balance iron. That steel, themicrostructure of which also is shown in the patent document, afterhardening and tempering contained totally 0.3 vol-% carbides of type M₂Cand M₆C, and 0.8 vol-% MC-carbides. The latter ones had an essentiallyspherical shape and the large sizes which are typical for high vanadiumsteels manufactured in a conventional way comprising ingot casting. Thesteel is said to be suitable for “plastic working”.

The above mentioned steel VANADIS® 4 has been manufactured since about15 years and has due to its excellent features reached a leadingposition on the market place for high performance cold work steels. Itis now the objective of the applicant to offer a high performance coldwork steel having still better toughness than VANADIS® 4 while otherfeatures are maintained or improved in comparison with VANADIS® 4. Thefield of use of the steel in principle is the same as for VANADIS® 4.

DISCLOSURE OF THE INVENTION

The above objectives can be achieved therein that the steel has thefollowing chemical composition in weight-%: 1.25-1.75 (C+N), however atleast 0.5 C, 0.1-1.5% Si, 0.1-1.5% Mn, 4.0-5.5 Cr, 2.5-4.5% (Mo+W/2),however max. 0.5% W, 3.0-4.5% (V+Nb/2), however max. 0.5% Nb, max. 0.3%S, balance iron and unavoidable impurities, and a microstructure, whichin the hardened and tempered condition of the steel, contains 6-13 vol-%of vanadium-rich MX-carbides, -nitrides and/or carbonitrides which areevenly distributed in the matrix of the steel, where X is carbon and/ornitrogen, at least 90 vol-%, of said carbides, nitrides and/orcarbonitrides having an equivalent diameter, D_(eq), which is smallerthan 3.0 μm, and preferably smaller than 2.5 μm in a studied section ofthe steel; and totally max. 1 vol-% of other, possibly existingcarbides, nitrides or carbonitrides. The carbides have a predominatelyround or rounded shape but individual, longer carbides may occur.Equivalent diameter, D_(eq) is defined in this context as Deq=2√A/π,where A is the surface of the carbide particle in the studied section.Typically, at least 98 vol-% of the MX-carbides, nitrides and/orcarbonitrides have a D_(eq)<3.0 μm. Normally, thecarbides/nitrides/carbonitrides also are spherodised to such a highdegree that no carbides have a real length in the studied sectionexceeding 3.0 μm.

In the hardened condition, the matrix consists essentially only ofmartensite, which contains 0.3-0.7, preferably 0.4-0.6% C in solidsolution. The steel has a hardness of 54-66 HRC after hardening andtempering.

In the soft annealed condition, the steel has a ferritic matrixcontaining 8-15 vol-% vanadium-rich MX-carbides, nitrides, and/orcarbonitrides, of which at least 90 vol-% have an equivalent diametersmaller than 3.0 μm and preferably also smaller than 2.5 μm, and max. 3vol-% of other carbides, nitrides and/or carbonitrides.

If otherwise is not stated, always weight-% is referred to concerningthe chemical composition, and vol-% is referred to concerning thestructural composition of the steel.

As far as the individual alloy elements and their mutual relationship,the structure of the steel and its heat treatment are concerned, thefollowing apply.

Carbon shall exist in a sufficient amount in the steel in order, in thehardened and tempered condition of the steel, to form, in combinationwith nitrogen, vanadium, and possibly existing niobium, and to somedegree also other metals, 6-13 vol-%, preferably 7-11 vol-% MX-carbides,nitrides or carbonitrides, and also exist in solid solution in thematrix of the steel in the hardened condition of the steel in an amountof 0.3-0.7, preferably 0.4-0.6 weight-%. Suitably, the content ofdissolved carbon in the matrix of the steel is about 0.53%. The totalamount of carbon and nitrogen in the steel, including carbon which isdissolved in the matrix of the steel plus that carbon which is bound incarbides, nitrides or carbonitrides, i.e. % (C+N), shall be at least1.25, preferably at least 1.35%, while the maximal content of C+N mayamount to 1.75%, preferably max. 1.60%.

According to a first preferred embodiment of the invention, the steeldoes not contain more nitrogen than what unavoidably will exist in thesteel because of take up from the environment and/or through suppliedraw materials, i.e. max. about 0.12%, preferably max. about 0.10%.According to a conceived embodiment, however, the steel may contain alarger, intentionally added content of nitrogen, which may be suppliedthrough solid phase nitriding of the steel powder which is used in themanufacturing of the steel. In this case, the main part of C+N mayconsist of nitrogen, which implies that said MX-particles in this casemainly consist of vanadium carbonitrides in which nitrogen is thesubstantial ingredient together with vanadium, or even consist of purevanadium nitrides, while carbon exists essentially only as a dissolvedingredient in the matrix of the steel in the hardened and temperedcondition of the steel.

Silicon is present as a residue from the manufacturing of the steel inan amount of at least 0.1%, normally in an amount of at least 0.2%.Silicon increases the carbon activity in the steel and thereforecontributes to affording the steel an adequate hardness. If the contentof silicon is too high, embrittlement problems may arise because ofsolution hardening, wherefore the maximal silicon content of the steelis 1.5%, preferably max. 1.2%, suitably max. 0.9%.

Manganese, chromium and molybdenum shall exist in the steel in asufficient amount in order to afford the steel an adequatehardenability. Manganese also has the function of binding those amountsof sulphur which may exist in the steel to form manganese sulphides.Manganese therefore shall exist in an amount of 0.1-1.5%, preferably inan amount of 0.1-1.2, suitably 0.1-0.9%.

Chromium shall exist in an amount of at least 4.0%, preferably at least4.5% in order to give the steel a desired hardenability in combinationwith in the first place molybdenum but also manganese. The chromiumcontent, however, must not exceed 5.5%, preferably not exceed 5.2%, inorder that undesired chromium carbides shall not be formed in the steel.

Molybdenum shall exist in an amount of at least 2.5% in order to affordthe steel a desired hardenability in spite of the limited content ofmanganese and chromium which characterizes the steel. Preferably, thesteel should contain at least 2.8%, suitably at least 3.0% molybdenum.Maximally, the steel may contain 4.5%, preferably max. 4.0% molybdenumin order that the steel shall not contain undesired M₄C-carbides insteadof the desired amount of MC-carbides. Higher contents of molybdenumfurther may cause undesired loss of molybdenum because of oxidation inconnection with the manufacturing of the steel. In principle, molybdenummay completely or partly be replaced by tungsten, but for this twice asmuch tungsten is required as compared with molybdenum, which is adrawback. Also any scrap which may be produced in connection with themanufacturing of the steel or in connection with the manufacturing ofarticles made of the steel, will be of less value for recycling if thesteel contains significant amounts of tungsten. Therefore tungstenshould not exist in an amount of more than max. 0.5%, preferably max.0.3%, suitably max. 0.1%. Most conveniently, the steel should notcontain any intentionally added tungsten, which according to the mostpreferred embodiment should not be tolerated more than as an impurity inthe form of a residual element from the raw materials which are used inconnection with the manufacturing of the steel.

Vanadium shall exist in the steel in an amount of at least 3.0% but notmore than 4.5%, preferably at least 3.4% and max. 4.0%, in order,together with carbon and nitrogen, to form said MX-carbides, nitridesand/or carbonitrides in a total amount of 6-13%, preferably 7-11 vol-%,in the hardened and tempered use condition of the steel. In principle,vanadium may be replaced by niobium, but this requires twice as muchniobium as compared with vanadium, which is a drawback. Further, niobiummay have the effect that the carbides, nitrides and/or carbonitrides mayget a more edgy shape and be larger than pure vanadium carbides,nitrides and/or carbonitrides, which may initiate ruptures or shippingsand therefore reduce the toughness of the material. Niobium thereforemust not exist in an amount exceeding 0.5%, preferably max. 0.3% andsuitably max. 0.1%. Most conveniently the steel should not contain anyintentionally added niobium. In the most preferred embodiment of thesteel, niobium therefore should be tolerated only as an unavoidablyimpurity in the form of a residual element from the raw materials whichare used in connection with the manufacturing of the steel.

According to the first embodiment, sulphur may exist as an impurity inan amount of not more than 0.03%. In order to improve the machinabilityof the steel, however, it is conceivable that the steel, according to anembodiment, contains intentionally added sulphur in an amount up to max.0.3%, preferably max. 0.15%.

At the manufacturing of the steel, first a bulk of molten steel isprepared, containing intended contents of carbon, silicon, manganese,chromium, molybdenum, possibly tungsten, vanadium, possibly niobium,possibly sulphur above impurity level, nitrogen in an unavoidabledegree, balance iron and impurities. From this molten material, powderis manufactured by the employment of nitrogen gas atomisation. The dropswhich are formed at the gas atomisation are cooled very rapidly, so thatthe formed vanadium carbides and/or mixed vanadium- and niobium carbidesdo not get sufficient time to grow but remain extremely thin—thicknessesof only a fraction of a micrometer—and get a pronouncedly irregularshape, which is due to the fact that the carbides are precipitated inremaining regions containing molten material in the networks of thedendrites in the rapidly solidifying droplets, before the dropletscompletely solidify to form powder grains. If the steel shall containnitrogen above the unavoidable impurity level, the supply of nitrogencan be performed by nitriding the powder, e.g., in the mode which isdescribed in SE 462 837.

After sieving, which is performed prior to the nitriding if the powdershall be nitrided, the powder is filled in capsules, which areevacuated, closed and subjected to hot isostatic pressing, HIP-ing, in amode which is known per se, at high temperature and high pressure;950-1200° C. and 90-150 MPa; typically at about 1150° C. and 100 MPa, sothat the powder is consolidated to form a completely dense body.

Through the HIP-ing operation, the carbides/nitrides/carbonitridesobtain a much more regular shape than in the powder. The great majority,with reference to volume, has a size of max. about 1.5 μm and a roundedshape. Individual particles are still elongated and somewhat longer,max. about 2.5 μm. The transformation probably is attributed to acombination of on one hand disintegration of the very thin particles inthe powder and on the other hand coalescence.

The steel can be used in the as HIP-ed condition. Normally, however, thesteel is hot worked subsequent to the HIP-ing through forging and/or hotrolling. This is performed at a start temperature between 1050 and 1150°C., preferably at about 1100° C. This causes further coalescence and,above all, globularisation (spheroidisation) of thecarbides/nitrides/carbonitrides. At least 90 vol-% of the carbides havea maximal size of 2.5 μm, preferably max. 2.0 μm after forging and/orhot rolling.

In order that the steel shall be able to be machined by means of cuttingtools, it first must be soft annealed. This is carried out at atemperature below 950° C., preferably at about 900° C., in order toinhibit growth of the carbides/nitrides/carbonitrides. The soft annealedmaterial therefore is characterized by a very finely disperseddistribution of MX-particles in a ferritic matrix, which contains 8-15vol-% MX-carbides, nitrides and/or carbonitrides of which at least 90vol-% has an equivalent diameter which is smaller than 3.0 μm and whichpreferably also is smaller than 2.5 μm, and max. 3 vol-% of othercarbides, nitrides and/or carbonitrides.

The tool is hardened and tempered when it has got its final shapethrough cutting type of machining. The austenitising is carried out at atemperature between 940 and 1150° C., preferably at a temperature below1100° C. in order to avoid undesirably great dissolution of MX-carbides,nitrides and carbonitrides. A suitable austenitising temperature is1000-1040° C. The tempering can be performed at a temperature between200 and 560° C., either as a low temperature tempering at a temperaturebetween 200 and 250° C., or as a high temperature tempering at atemperature between 500 and 560° C. TheMX-carbides/nitrides/carbonitrides are dissolved to a certain degree atthe austenitising such that they can be secondary precipitated inconnection with the tempering. The final result is the microstructurewhich is typical for the invention, namely a structure consisting oftempered martensite and, in the tempered martensite, 6-13 vol-%,preferably 7-11 vol-%, MX-carbides, nitrides and/or carbonitrides whereM essentially consists of vanadium and X consists of carbon andnitrogen, preferably substantially carbon, of which carbides, nitridesand/or carbonitrides at least 90 vol-% have an equivalent diameter ofmax. 2.5 an, preferably max. 2.0 μm, and totally max. 1 vol-% ofpossibly existing other types of carbides, nitrides or carbonitrides inthe tempered martensite. Prior to tempering, the martensite contains0.3-0.7, preferably 0.4-0.6% carbon in solid solution.

Further features and aspects of the invention is apparent from theappending patent claims and from the following description of performedexperiments.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of performed tests, reference will be madeto the accompanying drawings, in which:

FIG. 1 shows the microstructure at a very large magnification of a metalpowder of the type which is used for the manufacturing of the steelaccording to the invention,

FIG. 2 shows the microstructure of the same steel material afterHIP-ing, however at a smaller magnification,

FIG. 3 shows the same steel material as in FIG. 2 after forging,

FIG. 4 shows the microstructure of a reference material after HIP-ingand forging,

FIG. 5 shows the microstructure of the steel according to the inventionafter hardening and tempering,

FIG. 6 shows the microstructure of the reference material afterhardening and tempering,

FIG. 7 is a diagram showing the hardness of a steel according to theinvention and the hardness of a reference material versus theaustenistising temperature,

FIG. 8 shows the hardness of the steel according to the invention and ofthe reference material, respectively, versus the tempering temperature,and

FIG. 9 shows hardenability curves for a steel of the invention and for areference steel.

DESCRIPTION OF PERFORMED TESTS

The chemical composition of the tested steels are stated in table 1. Inthe table, the content of tungsten is stated for some of the steels,which content exists in the steel as a residue from the raw materialswhich are used for the manufacturing of the steel and is therefore anunavoidable impurity. The sulphur, which is stated for some of thesteels, also is an impurity. The steel contains other impurities aswell, which do not exceed normal impurity levels and which are notstated in the table. The balance is iron. In table 1, steels B and Chave a chemical composition according to the invention. Steels A, D, Eand F are reference materials; more particularly of type VANADIS® 4.

TABLE 1 Chemical composition in weight-% of tested steels Steel C Si MnS Cr Mo W V N A 1.56 0.92 0.40 n.a. 8.15 1.48 n.a. 3.89 0.067 B 1.550.89 0.44 n.a. 4.51 3.54 n.a. 3.79 0.046 C 1.37 0.38 0.37 0.015 4.813.50 0.10 3.57 0.064 D 1.55 1.06 0.44 0.015 7.95 1.59 0.14 3.87 0.107 E1.55 1.04 0.41 0.016 7.95 1.49 0.14 3.72 0.088 F 1.53 1.05 0.40 0.0157.97 1.50 0.06 3.84 0.088 n.a. = not analyzed

Bulks of molten steel with the chemical compositions of the steels A-Faccording to table 1 where prepared according to conventional, meltmetallurgical technique. Metal powders where manufactured of the moltenmaterial by nitrogen gas atomisation of a stream of molten metal. Theformed droplets were cooled very rapidly. The microstructure of steel Bwas examined. The structure is shown in FIG. 1. As is apparent from thisfigure, the steel contains very irregularly shaped, very thin carbides,which have been precipitated in the remaining regions containing moltenmetal in the net work of the dendrites.

HIP-ed material was also produced at a small scale of powders of steelsA and B. 10 kg powder of each of the steels A and B were filled in metalsheet capsules, which were closed, evacuated and heated to about 1150°C. and were then hot isostatic pressed (HIP-ed) at about 1150° C. and apressure of 100 MPa. At the HIP-ing operation the originally obtainedcarbide structure of the powder was broken down at the same time as thecarbides coalesced. The result which was obtained for the HIP-ed steel Bis apparent from FIG. 2. The carbides in the HIP-ed condition of thesteel have got a more regular shape, which is closer the spherodisedshape. They are still very small. The great majority, more than 90vol-%, have an equivalent diameter of max. 2 μm, preferably max. about2.0 μm.

Then the capsules were forged at a temperature of 1100° C. to dimension50×50 mm. The structure of the material of the invention, steel B, andof the reference material, steel A, after forging, are apparent fromFIG. 3 and FIG. 4, respectively. In the material of the invention thecarbides in the form of essentially spherodised (globular) MC-carbideswere very small, still max. about 2.0 μm in size, in terms of equivalentdiameter. Only few carbides of other types, more specificallymolybdenum-rich carbides, probably of type M₆C, could be detected in thesteel of the invention. The total amount of these carbides was less than1 vol-%. In the reference material, steel A, FIG. 4, on the other handthe volume fractions of MC-carbides and chromium-rich carbides of typeM₇C₃ were approximately equally large. Further, the carbide sizes wereessentially larger than in the steel of the invention.

Thereafter full scale test were performed. Powders were produced ofsteels having chemical compositions according to table 1, steels C—F, inthe same way as has been described above. Blanks having a mass of 2 tonswere produced of steel C of the invention by HIP-ing in a mode which isknown per se. Thus the powder was filled in capsules which were closed,evacuated, heated to about 1150° C. and hot isostatic pressed at thattemperature at a pressure of about 100 MPa. Of the reference steels D, Eand F, there were produced HIP-ed blanks according to the applicant'smanufacturing praxis for steel of type VANADIS® 4. The blanks wereforged and rolled at about 1100° C. to the following dimensions; steelC: 200×80 mm, steel D: 152×102 mm and steel E: Ø 125 mm.

Samples were taken from the materials after soft annealing at about 900°C. The heat treatment in connection with hardening and tempering isstated in table 2. The microstructures of steels C and F were examinedin the hardened and tempered condition of the steels and are shown inFIG. 5 and FIG. 6. The steel of the invention, FIG. 5, contained 9.5vol-% MC-carbides in the matrix, which consisted of tempered martensite.Any carbides and/or carbonitrides of other type than the MC-carbideswere difficult to detect. Anyhow, the amount of such possible, furthercarbides, e.g., M₇C₃-carbides, anyhow was less than 1 vol-%. Occasionalcarbides having an equivalent diameter larger than 2.0 μm could bedetected in the steel of the invention in the hardened and temperedcondition of the steel, but no ones were larger than 2.5 μm.

The reference material, steel F, FIG. 6, contained totally about 13vol-% carbides, thereof about 6.5 vol-% MC-carbide and about 6.5 vol-%M₇C₃-carbides, in the hardened and tempered condition of the steel.

The hardness obtained after the heat treatment stated in table 2 is alsostated in table 2. Steel C according to the invention achieved ahardness of 59.8 HRC in the hardened and tempered condition, while thereference steels D and E got a hardness of 58.5 and 61.7 HRC,respectively.

The hardnesses of the steels C and D that could be achieved afterdifferent austenitising temperatures and tempering temperatures werealso investigated. The results are apparent from the curves in FIG. 7and FIG. 8. Steel C of the invention, FIG. 7, had a hardness which wasvery little dependent on the austenitising temperature. This isadvantageous, because it allows a comparatively low austenitisingtemperature. 1020° C. turned out to be the most suitable austenitisingtemperature, while the reference steel had to be heated to about1060-1070° C. in order to achieve maximal hardness.

As is apparent from FIG. 8, steel C of the invention also had anessentially better tempering resistance than the reference steel D. Apronounced secondary hardening was achieved by tempering at atemperature between 500-550° C. The steel also gives a possibility tolow temperature tempering at about 200-250° C.

The impact toughness of steels C and D was examined. The absorbed impactenergy (J) in the LT2-direction was 102 J for steel C according to theinvention, i.e. an extremely great improvement as compared with thehardness 60 J which was obtained for the reference material, steel D.The test specimens consisted of milled and ground, un-notched test barshaving the dimension 7×10 mm and the length 55 mm, hardened tohardnesses according to table 2.

During wear tests there were used test specimens having the dimension Ø15 mm and the length 20 mm. The test was performed via pin-to-pin testusing SiO₂ as abrasive wear agent. Steel C of the invention had a lowerwear rate, 8.3 mg/min, than the reference material, steel E, for whichthe wear rate was higher, 10.8 mg/min, i.e the wear resistance of thatmaterial was lower.

TABLE 2 Un-notched impact energy Hardness in the LT2- Wear rate SteelHeat treatment (HRC) direction (J) (mg/min) C 1020° C./30 min + 59.8 1028.3 550° C./2 × 2 h D 1020° C./30 min + 58.5 60 525° C./2 × 2 h E 1050°C./30 min + 61.7 10.8 525° C./2 × 2 h

The hardenability of steel C of the invention and of a steel of typeVANADIS® 4 manufactured in full scale production were examined. Theaustenitising temperature, TA, in both cases was 1020° C. The sampleswere cooled at different cooling rates, which were controlled by more orless intense cooling by means of nitrogen gas from the austenitisingtemperature, TA=1020° C., to room temperature. The periods required forcooling from 800° C. to 500° C. were measured as well as the hardness ofthe specimens which had been subjected to varying cooling rates. Theresults are stated in table 3. FIG. 9 shows the hardness versus the timefor cooling from 800° C. to 500° C. As is apparent from this figure,which shows the hardenability curves for the examined steels, the curvefor steel C of the invention lies at a significantly higher level thanthe curve for the reference steel, which means that the steel of theinvention has an essentially better hardenability than the referencesteel.

TABLE 3 Hardenability measurement; TA = 1020° C. Cooling period betweenVANADIS ® 4 Steel C 800° C. and 500° C. (Sec) Hardness (HV10) Hardness(HV10) 139 767 858 415 — 858 700 734 858 2077 634 743 3500 483 606 7000274 519

1. Cold work steel in the hardened and tempered condition, manufacturedpowder metallurgically, having a wear rate of 8.3 mg/min, and having thefollowing chemical composition in weight-%: 1.37% C 0.38% Si 0.37% Mn4.81% Cr 3.50% Mo 0.1% W 357% V max 0.3% S 0.064% N balance iron andunavoidable impurities; and a microstructure which in the hardened andtempered condition of the steel contains 6-13 vol-% of vanadium-richMX-carbides,-nitrides and/or carbonitrides which are evenly distributedin the matrix of the steel, where X is carbon and/or nitrogen, at least90 vol-% of said carbides, nitrides and/or carbonitrides having anequivalent diameter, D_(eq), which is smaller than 3.0 μm; and a totalmax. 1 vol-% of other existing carbides, nitrides or carbonitrides. 2.Steel according to claim 1, wherein the matrix of the steel in thehardened condition of the steel essentially only consists of manensite,which contains 0.3-0.7% C in solid solution.
 3. Steel according to claim1, wherein at least 98 vol-% of said MX-carbides, nitrides and/orcarbonitrides have an equivalent diameter, D_(eq), which is smaller than3.0 μm.
 4. Steel according to claim 2, wherein the steel, afterhardening and tempering., has a hardness of 54-66 HRC.
 5. Steelaccording to claim 4, wherein the steel, after hardening and tempering,has a hardness of 60-63 HRC.
 6. Steel according to claim 1, wherein thesteel contains 7-11 vol-% MX-carbides, nitrides and/or carbonitrides,where M consists substantially of vanadium and X is carbon and/ornitrogen.
 7. Steel according to claim 6, wherein the steel contains 9.5vol-% MC-carbides.
 8. Steel according to claim 1, wherein the steel ismanufactured powder metallurgically by manufacturing a powder of amolten metal and hot isostatic pressing the powder into a consolidatedbody.
 9. Steel according to claim 8, wherein the hot isostatic pressingis performed at a temperature between 950 and 1200° C. and at a pressurebetween 90 and 150 MPa.
 10. Steel according to claim 8, wherein thesteel after hot isostatic pressing has been hot worked, starting at astarting temperature between 1050 and 1150° C.
 11. Steel according toclaim 9, wherein the steel is hardened from a temperature between 940and 1150° C. and tempered at a temperature between 200 and 250° C. or ata temperature between 500 and 560° C.
 12. Steel according to claim 1,wherein at least 90 vol-% of the MX-carbides, nitrides and/orcarbonitrides have a maximal extension of 2.0 μm after hot isostaticpressing, hot working, soft annealing, hardening and tempering of thesteel.
 13. Cold work steel according to claim 1 wherein the steel has aferritic matrix containing 8-15 vol-% MX-carbides, nitrides and/orcarbonitrides, of which at least 90 vol-% have an equivalent diameterwhich is smaller then 3.0 μm, and max. 3 vol-% of other carbides,nitrides and/or carbonitrides after soft annealing and prior tohardening and tempering of the steel.
 14. A tool for shearing, cuffing,blanking and/or metal forming of metallic working material in a coldcondition of the material, or for pressing metal powder manufacturedfrom a steel as claimed in claim
 1. 15. Steel according to claim 2,wherein 0.4-0.6% C is in solid solution.
 16. Steel according to claim 3,wherein said equivalent diameter, D_(eq), is smaller than 2.5 μm. 17.Steel according to claim 4, wherein after hardening and tempering thehardness is 58-63 HRC.
 18. Cold work steel according to claim 13 whereinthe equivalent diameter is smaller than 2.5 μm.